I am trying to fix a bike which has no shifter on the handlebar and on the back wheel has a shimano nexus sg7c25 (7 speed), i am trying to figure out how to install a cable so i can change gears with a shifter (that I will buy).
I guess there is some pieces missing, I enclose some photos where you can take a look.

May be is missing the cassette joint? How to install it? I don't know if it is only one piece missing there ( I'd like to know if you have a guide on how to install a cable in a shimano nexus of this kind.

First off if I have read the site you gave for sheldonbrown then I think you are missing a few of the parts you need. And I think you have the 8 speed if I read the page right. Not much help I know but stick around here and I am sure there will be some one here that can and will help you out on this. You can do a google search for a video on this model you have. I have seen a video on this very drive train before. So do a search it want be real easy to find but I do know its out here on the web. I am sure someone will give you the link to that too. Maybe this will give you the hope you need. I do try to help when I can. Just this time around I'm not doing so good at it lol.

the nexus SG-7c25 should be 7 speeds right, the model is written on it? I guess also that the pieces i missed are that shown in the video, (Cassette Joint, 3 pieces) than i have to install the cable and the shifter on the handlebar.

Is there somebody that confirms that I miss "only" the Cassette joint (see figures enclosed) and if you can help with installing the cable and finally, how to adjust the speeds with the shifter?
Thanks a lot,

Hello, I'd like to know what is exactly the part that is moving when shifting the gears.
Looking at the photo below, is the point A or B rotating when pulling the cable? if not what is the part that is rotating there?

To answer the first question on how to install the cable: Using the pdf diagram that Bill provided; (parts are numbered)parts labeled 37 are the parts in the youtube video, but the video fails to show part 40 or shimano part CJ-NX10 the inner cable fixing bolt. That part is where the cable fixes to on part 39 and the outer cable stops on part 39 (with the built-in outer cable stop). The yellow dots are used to tune the shifter, and the dots should align in 4th gear (youtube).
So your last question is hard to answer because the internal hub works a lot differently than the traditional set up. The shifter changes the inner hubs relation to the internal gears.